I'd love the idea of a switch to iPod Touches. If anyone has ever been to an Apple Store, you've seen the attachment they have hooked onto their iPod Touches that lets you scan barcodes. However, that's Apple, and there would be many more headaches with an iPod Touch.
1) The battery. It's not removable. To replace the battery after it's worn down means replacing the device itself. And say the battery is close to dying in the middle of your shift, but there's no more iPods available for use. You can't just switch it out.
2) The redesigns. The external scanner case attachment that Apple has at their stores are designed for the current generation iPod Touch (I would imagine). Say #1 happens here, and you have to replace the iPod Touch down the road. The model that works with the scanners has been turned into a device the thickness of a credit card, and isn't compatible with the previous generation scanner case!
3) Software. Yeah, you could just stick with the current set up and have it access the same HTML page that our current PDAs access. But those pages are designed for our PDAs... they're not designed for Mobile Safari.
4) Theft. Think about why no one steals our current PDAs. Because they're huge, clunky, and not very appealing machines to have other than at our job. And even if you don't mean to steal anything, iPod Touches are easily pocketable and would wind up at home by accident countless times.
As much as I'd like the switch (and how good of an idea it would seem to be), it's just not feasible. One thing I don't get is how our current PDAs cost so damn much. Someone is making a killing off retail IT budgets.